GB2035981A

GB2035981A - Silicon nitride-based sintering composition

Info

Publication number: GB2035981A
Application number: GB7938602A
Authority: GB
Original assignee: Battelle Memorial Institute Inc
Current assignee: Battelle Memorial Institute Inc
Priority date: 1978-11-08
Filing date: 1979-11-07
Publication date: 1980-06-25
Also published as: DE2945146A1; US4264547A; GB2035981B; JPS55104976A; JPS5939770A; JPS6220150B2; JPH0147428B2; DE2945146C2

Description

1

SPECIFICATION Silicon Nitride-based Sintering Composition

GB 2 035 981 A 1 This invention relates to powder compositions based on silicon nitride Si3N4. The invention also relates to the manufacture of articles formed from silicon nitride by thermal sintering of such compositions.

Silicon nitride is a very hard material which is suitable for manufacturing parts having high mechanical strength at high temperature (shafts, gas turbine blades, parts in contact with liquid metals, block bearings, ball bearings, sealing segments etc.), provided that its porosity is low. The higher the porosity of this material, the less it resists breakage forces and hot oxidation corrosion. Si,N4 of very low porosity, which is suitable for the aforesaid applications, can be made by hot anisotropic mechanical pressing. By this method, compact Si, N4 is obtained in the form of blocks, which are very costly to convert into mechanical parts because of the extreme hardness of the material, the special tools (diamond wheels) required for machining it, and the slowness of this work. Active attempts have been made during recent years to directly form parts by moulding or stamping powder compositions based on Si,N4, followed by hot sintering of the shaped parts under an inert atmosphere. In doing this, 15 the following three basic factors have proved important: the addition of densification aids, the use of powders of fine particle size (of the order of 1 to a few gm) and, during sintering, the use of a relatively high nitrogen pressure, of the order of 2 to 50 atmospheres. By means of these improvements, densification levels of the order of 95 to 97% of the theoretical density (3.19 gm/ce) are now expected (3.03-3.097 g/CM3).

The most commonly used densification aids include MgO (5%); A1203+Y203 (10-50%); BeO(1.25%)+MgO(3.75%); BeO(1.25%)+MgO(3.75%)+Ceo2(5%), etc.

The most important publications in this field include, for example: "Pressureless sintered silicon carbide" by 1. Oda, M. Kaneno and N. Yamamoto, Research and Development Laboratory, NGK Insulators Ltd., Mizuho, Nagoya, Japan; Yogyo Kyoksi Shi 1976, 84 (8), 356-60 (Japan): Japan J. 25 Mater. Sci 1976, 11 (6), 1103-7; Japanese Kokai patent specification 77 47,015; Yogyo Kyoksi Shi

1977; 85 0,408-12; G. R. Terwilliger & F. F. Lange, Journal of Materials Science 10 (1975), 1169 1174; USP 3,992,497 and "Sintering of silicon nitride" by D. J. Rowcliffe & P. J. Jorgensen, Stanford Research Institute, Menlo Park, California.

U. S. Patent 3,953,221 (Lange) concerns the pressureless sintering of powder mixtures Of S6N4, 30 A120, and M90. In this mixture, the ratio of the sintering aids to SI.N4 is never below 20:80 and the density of the obtained article does not appear to exceed about 95-96% of theory.

U. S. Patent 4,073,845 (Sylvania) concerns a powder mixture consisting of Si3N4 and, optionally, MgO and A1203, for the pressureless hot sintering of silicon nitride articles, the density of which may reach about 96% of theory with a rupture modulus in the vicinity of 105 psi=79 kg/MMI. The preferred ' 35 amount of MgO is about 5% or less and the particle size of the S04 IS less than 3 U and, preferably, 0.5-1 A; nothing is stated about the possible usable quantities of A120.' C. A. 80, 231, 111869 m (Japanese Patent Application No. 731 79,216 Tokyo Shibaura) concerns the hot sintering of S6NCA1203 mixtures (ratio 99.9:0.1-80:20) after cold compression moulding. S6N4 particles are 0.5,u and A1203 particles are 0.2 ju. MgO is not indicated and the bending 40 strength of the sintered articles is only in the range of 55 kg/cM2.

German Patent publication DOS 23 53 093 (Toyota) concerns the hot pressureless sintering of mixtures of S6N4 with 8 to 40% of mixtures of metal oxides, for example a mixture of MgO and A1203.

The method provides articles having densities reaching 3.13 and rupture moduli reaching 65 kg/mM2.

However, from the examples in this reference, it is clear that the highest performances are obtained 45 when the MgO and the A1203 are first reacted together to produce spine], the latter having to be subsequently finely ground before mixing with the S6N4.

British Patent No. 1,485,384 (Lucas) concerns a process for sintering S6N, together with a first and a second metal oxide, such metal oxides (not specifically named) being selected for having the properties of forming iow-melting silicates with the silica possibly present within the S1,1\14. An example 50 indicates that the metal oxides are MgO and Fe,0, It would be desirable to attain even higher densification levels, in order to reduce the porosity levels, as the existing pores can be a source of cracks in the final pieces. To attain this, the theoretical density of 3.19 g/cml should be approached as closely as possible, while at the same time using as small a proportion of densification aids as possible in order to preserve the most favourable mechanical 55 properties in the sintered material, such as hardness, resistance to bending, resistance to tensile stress and resistance to breakage, particularly at high temperature. If powders are used containing relatively high proportions of densification aids, the mechanical properties, which are good at low temperature, can become mediocre at high temperature. Thus, if the aforesaid BeO+MgO+ Ceo2 is used, the modulus of rupture fails from 83 kg per/m M2 at ambient temperature to 4 kg/mml at 14001)C. On the 60 other hand, Y201 is expensive, and BeO is undesirable because of its toxicity. Moreover, it is economically preferable to sinter at atmospheric pressure rather than at a higher pressure, because in this manner the problems relative to the strength of materials at high temperature under pressure, such as the sealing of furnaces operating in the region of 1 500-20001T, can be avoided.

2 GB 2 035 981 A 2 Recently, pressureless sintering of a Si^ powder containing 10 mol % of spinel (Mg aluminate) for 4 hours at 18501C has been described, leading to a clensification exceeding 96% and the formation of a material having an ultimate bending strength of 72 kg/mml, which is high (see Yogyo Kyokai Shi 1976, 84 (10), 508-12). It is also interesting to note that according to this reference, it is not possible 5 to obtain equivalent results by using MgO and A1201 powders instead of spine].

In fact, we have now found that these densification aids can be used in pressureless sintering to enable products to be obtained having a quality better than or at least equivalent to those of the prior art, and in particular having a density very close to the theoretical density.

The present composition contains powdered Si,N, plus magnesium oxide and aluminium oxide as densification aids; the particle size of the Si^ used is less than 1 Am, the particle size of the densification aids is less than the particle size of the S'31\14, the total amount of the densification aids does not exceed 6% by weight of the composition, and the MgO:A1,0, weight ratio lies between 10:1 and 1:3.

The above-mentioned U. S. Patent 4073845 strongly stresses the importance of having, in the starting powder, a definite ratio of crystalline S13N4 to amorphous S1, 1\14 (ratio from 5% to 6%) for obtaining the optimum properties of the sintered article. For instance, the property versus % crystallinity curves have a maximum when the % crystallinity of the S6N, is about 20%. This behaviour does not agree at all with the results of the present invention since, in the invention, satisfactory sintered articles can be obtained from any form of Si3N4 (a, P, amorphous, or mixtures thereof). Further, the properties of the sintered articles are definitely better in the present invention, which can provide 20 densities of 98-98.5%. Thus, the products of U. S. Patent 4073845 and that of the invention are not comparable.

In contrast to the above-mentioned DOS 23 53 093, if the factor relating to the size ratio of the Si3N4 and the sintering aid particles is correctly implemented in accordance with the present invention, high-performance sintered articles are obtained without having to go through a preliminary spinel 25 forming step. This is economically important and constitutes a marked advantage of the invention over DOS 23 53 093.

The preferred method for using the composition according to the invention for manufacturing sintered Si3N4 parts of high strength is characterised by the following steps:

a) the powder is compacted in the cold state into the form of the desired object, b) this moulded object is subjected in its cold state to an isostatic pressure exceeding 1 T/cM2 c) the object is heated under reduced pressure in order to degas it, and d) the object is heated for 2 to 20 minutes between 1650 and 18300C under an essentially 1 nitrogen atmosphere, this latter operation giving rise to the required sintering and densification.

This method is extremely advantageous, because by taking account of the contraction during 35 densification (of the order of 40 to 60% by volume) the object can be formed of approximately the required proportions, for example by moulding or stamping, so enabling further machining to be reduced to a strict minimum. It is also possible to grind the part before sintering (green machining) or after pre-sintering at around 14001C. This method is m ore economical than known methods, because it does not require, as the latter methods require, any complicated heating programme during the sintering operation (for example graduated heating), and the heating time required is very short.

Preferably, after step a), which is carried out by usual known means, step b) is carried out at 6 - T/crn'. To attain this, the moulded object can for example be wrapped in a flexible plastics sheet and the whole subjected to a hydrostatic pressure by means of a liquid such as oil in a suitable press.

Alternatively, the object can be moulded in a rubber mould, the mould then being pressed in a piston press, the forces due to the pressure then becoming distributed uniformly in all directions by way of the material constituting the mould. After cold pressing and removal from the mould, the preformed object (green) is obtained, constituted of agglomerated powder having a---green- density of the order of 1.4 to 1.8 gm/cc, this value depending on the particle size and the crystalline state (a, P, or amorphous form) of the Si,N4 used for the formulation of the starting compo sition.

Steps c) and d) can be carried out as follows: the green object is placed in a graphite crucible provided with a tight fastener (for example of screwed type), to reduce any N, losses by high temperature decomposition, and in order to prevent the green object from coming into direct contact with the crucible walls during heating, it is embedded in a powder which is inert at high temperature.

The powder used can be uncompacted silicon nitride, possibly containing boron nitride to prevent the 55 Si,N4 of this mixture sintering at the temperature used for sintering the part, and thus to facilitate the stripping of the part after cooling. For degassing it is then heated for about a half hour to one hour at around 800 to 1 OOOOC under 10-3 to 10-4 Torr. A protecting atmosphere (for example N,+ 1 % H,) is then introduced, the temperature is raised rapidly to the sintering point, this temperature is maintained for the required time, and finally the whole is allowed to cool. The heating time and sintering temperature are related in the sense that the time is shorter, the higher the temperature. Preferably, heating is carried out for about 15 minutes around 1 7500C. These conditions are given here only by way of example, but demonstrate the economical importance of the present method. If required, after sintering, the part can be annealed at a temperature (for example of the order of 1 60WC) which modifies its microstructure and improves its mechanical properties.

J X 4 z 3 GB 2 035 981 A 3 To prepare the powder mixture constituting the composition of the invention, commercially available ingredients can certainly beused provided that their particle size lies within the aforesaid range, they being mixed intimately by the usual means. If the powders used are too coarse, they can be previously ground down in suitable crushers or grinders, again by known means. Alternatively, the mixture can be made with such powders and the mixture then pulverised in such a way that after pulverising, the particles of its constituents have a suitable particle size lying within the aforesaid range. Preferably, the starting materials are Si,N4 with a particle size not exceeding 0.5 Am (specific surface 7-10 m2/9) and MgO having a particle size of the order of 0.05 to 0. 1 g, these then being pulverised together in a mill containing aluminium oxide balls, so that as these balls wear down, the required proportion of aluminium oxide becomes incorporated into the mixture in a finely ground state. 10 Obviously, a mixture of Si,N, MgO and A1,0, in the required proportions can be initially used, and pulverised in a mill, for example of steel or tungsten carbide. In this case, it is necessary to wash the ground powder with dilute acid (for example HCI) in order to remove from it any traces of iron transferred from the mill, after which it is dried. Preferably, at the moment of moulding the object, the composition according to the invention contains 2 to 5% of MgO and 0.2 to 1 % of A12o., the most 15 favourable composition being around 5% of MgO and 1 % of A1201. Obviously, if required, the composition can also contain other metal oxides in addition, in particular those described in the prior art. However, the proportions of additives are kept as low as possible in order for the proportion of S'31\14 in the final sintered product to be very high, and its porosity to be kept at a minimum level.

In order to attain effective pulverisation and dispersion of the constituents of the powder of the 20 present composition, it is advantageous to carry out this operation in a viscous liquid which gives a pasty consistency to the mixture, and which moreover has the advantage of protecting the Si,N4 from the air (formation of Si0J. The liquid used can be an organic liquid, the type of which is not critical, hydrocarbons and alcohols being very suitable, with preference for the use of a mixture of petroleum ether and tertiary butanol. After pulverising, the powder is carefully dried, preferably in an oven at 25 1 501C, then under vacuum.

The composition according to the invention, and its method of use for manufacturing parts by sintering, lead to products having excellent physical properties which are equivalent to or rather better than those of similar known products. The densities obtained after sintering can easily reach 3.10 to 3.15 g/CM3, corresponding to 97-98.5% of the theoretical density calculated by taking account of the 30 presence of the densification aids (3.20 g/cml). Generally, the ultimate bending strength of the products (see the measurement description in the following particular description) lies between 70 and kg/m M2, although exceptionally values up to 104 kg/m M2 have been measured. It should be noted that such results are nearly independent of the crystalline nature of the S'31\14 initially used. This aspect is illustrated by the accompanying drawing which shows the variations in the density of the product 35 against the sintering temperature (15 minutes) for a composition containing 5% of MgO of 1 % of A1203 in addition to the S6N4. The curve a represents the use of a-S'3N4, the curve P represents the use of S6NO and the curve A represents the use of amorphous Si,N4. It can be seen from these curves that optimum density values are attained around 1750' for a-S'3N4, around 18000 for P-S04, and at a substantially intermediate value for the amorphous composition. The three forms thus tend towards an 40 identical state during the sintering operation.

The examples given hereinafter illustrate the invention in a more detailed manner.

Example 1

In a 500 mi capacity mill containing aluminium oxide bells, 95 g of amorphous Si,N4 (Sylvania SN-402; particle size 0.3 Am, 11 m'/g) were mixed and pulverised for 5 days with 5 g of MgO (Merck 45 5865, 0.05 Am) and 200 mi of a 3:1 mixture of petroleum ether (B.P. 35- 45IC) and tertiary butanol.

After eliminating the solvent by drying overnight at 1501 at atmospheric pressure then under vacuum, the powder was analysed and found to contain 1 g of very finely ground powdered A1203, this amount of aluminium oxide resulting from the wear of the balls of the grinding mill during pulverisation. The powder was moulded into the form of a mechanical part in a rubber mould and this mould was then 50 subjected (as described above) to an isostatic pressure of 6 T/cM2 in a hydraulic press. The green part thus obtained (d=1.4) was then placed in a graphite crucible provided with a screwed fastener, embedding the part in a 1 A weight mixture of Si3N4 and BN. Degassing was then carried out for about one hour at 10001, and the temperature was then rapidly raised, sintering then being carried out for 15 minutes at one of the temperatures (between 1560 and 18 1 OIC) shown in Table 1 below. After cooling, the density of the sintered object was then measured by picnometry. The results obtained for the sintered samples at various temperatures are summarised in Table 1, showing that the optimum density (3.04 g/M3) corresponds to a temperature of 1 7800C and a contraction (by volume) of about 60%.

4 GB 2 035 981 A 4 Table 1 Sintering of amorphous silicon nitride with 5% of M90 and 1 % of aluminium oxide-heating time 15 minutes-influence of temperature Density g1CM3 sintered Sample No. Temperature 'C green object Contraction % 5; 6-2 1560 1.49 1.92 29.5 6-1 1620 1.44 2.13 38.5 6-3 1687 1.38 2.60 52.6 6-4 1750 1.33 2.73 53.6 10 6-5 1780 1.37 3.04 59.1 66 1810 - 2.92 - The variations in the density of the green part are due to the inevitable variations in the degree of filling of the mould. It has been found that these variations do not influence the conditions or results of the sintering.

Example 2

The method of Example 1 was followed, but the amorphous Si,N, was replaced in the composition by an identical quantity of a-S6N4 (Hermann Starck, Berlin, particle size 0.5,um). After compacting, green parts were obtained having a density close to 1.90 gm/cc. The sintering conditions (15 minutes) are shown in Table 11 together with the results.

Under these conditions, the maximum density was obtained for the sample sintered at 17500C.

The density of 3.15 g/cm3 is distinctly greater than the values given for the materials obtained by pressureless sintering following the prior art. Depending on the samples, the modulus of rupture at ambient temperature varied from 72 to 94 kg/mml, which is practically equivalent to the values obtained for materials densified by hot pressing. At 12500C, the bending strength was still 42 kg/m M2.25 Table 11 Sintering a silicon nitride with 5% MgO and 1 % A1103 for 15 minutes: influence of temperature- Density gIcm3 sintered Sample No. Temperature 'C green object Contraction % 30 1-2 1680.1.90 3.11 39.6 1-3 1750 1.91 3.15 40.1 1-4 1810 1.91 3.06 38.6 It should be noted that the modulus of rupture is measured in the following manner:

a bar of sintered S'31\14 is cut having a width of p and a thickness of e, and is placed horizontally on 35 two support points separated by a distance D, less than the total length of the bar. A central vertical force F is applied between the support points, and the value of F necessary for rupturing the bar is noted. The rupture modulus a is given by 1.5xF(kg)xD(mm) e(mm) Xp2() Example 3

The method of example 2 was followed using a composition identical to that described in this example, and sintering was carried out at 17501C for different times as shown in Table Ill. It can be seen from the results also given in Table Ill that extremely short sintering times (of the order of only two minutes) gave high-density sintered products. It can also be seen that heating times in excess of 45 20 minutes are unfavourable, as a certain degree of decomposition then takes place.

V.

A GB 2 035 981 A 5 time.

i Table Ill Sintering a silicon nitride with 5% MgO and 1 % A1203 at 1 750OC-influence of the sintering Density g1CM3 Sintering time sintered 5 Sample No. minutes green object Contraction % 1-6 2 2.05 3.10 34.4 1-3 15 1.91 3.15 40.1 1-8 45 1.97 2.87 31.9 Example 4

97 g of a silicon nitride (Starck 1316, particle size 0.5 urn; 7-10 g/M2) were mixed for five days in an aluminium oxide ball mill with 3 g of magnesium oxide (Merck 5865, particle size 0.05 pm) and 200 crn' of a 3:1 mixture of petroleum ether (B.P. 35-451) and tertiary butanol, this introducing 1 % of finely ground Al203. After eliminating the organic solvent, the powder was pressed lsostatically at 6 T/cM2, sheltered from the air (density of the green part about 1.90 g/cml). The sample was then placed 15 in a closed graphite crucible as described in example 1, and degassed at 1 0001C under a vacuum of 10-1 Torr, after which it was heated for 15 minutes at the temperatures indicated in Table IV. Under these conditions, maximum density was obtained in the case of the sample heated to 17501C. Although the value of 3.13 g/cm' is slightly less than that obtained with 5% of magnesium oxide, it is higher than the values given for the materials previously obtained by pressureless sintering in accordance with the price art. The modulus of rupture determined at ambient temperature varied from 50 to 70 kg/cm', depending on the sample.

Table IV Sintering a silicon nitride with 3% MgO and 1 % A1203 for 15 minutes-influence of temperature Density g1CM3 25 sintered Sample No. Temperature 'C green object Contraction % 8-3 1720 1.93 3.10 38.8 8-1 1750 1.81 3.13 43.0 8-2 1780 1.88 3.10 40.2 30 Example 5

The method of example 4 was followed, but P-S6N4 was used as the silicon nitride (MRC-2286 of Material Research Chemicals, U.S.A., particle size 0.5 pm).

The sintering conditions are given, with the results, in Table V, which shows that the maximum density (3.13 g/CM3) was obtained for 15 minutes of heating at 1813'C. This density is greater than 35 the density of the products obtained by the prior art for pressureless thermal pressing.

Table V

Sintering of P silicon nitride with 5% MgO and 1 % A1203 for 15 minutesinfluence of temperature.

Density g1CM3 40 sintered Sample No. Temperature OC green object Contraction % 2-2 1680 1.98 2.90 33.9 2-1 1750 2.00 2.98 33.2 2-4 1810 1.95 3.13 40.15 45 2-3 1870 1.94 3.05 36.74 Example 6 (Comparative example-influence of the amount of aluminium oxide).

For comparison with the results of example 4, a sintering powder was prepared containing 97 g of a-S04 and 3% of MgO. This powder was pulverised for only 5 hours in the aluminium oxide ball 50 mill instead of 5 days as in the preceding examples. After this period, the powder contained only about 0.05% of aluminium oxide (traces). This powder was compacted, and samples were prepared as described in the preceding examples (density of the green parts 1.97 g/CM3), and these samples were sintered for 15 minutes at 1730 and 1 7701C. In the two cases, the final densities were only 2.91 g/CM3, while the corresponding values obtained previously (see example 4) were close to 3.10 g/cm3, 55 6 GB 2 035 981 A 6 so demonstrating the importance of maintaining the quantity of aluminium oxide relative to the quantity of MgO within the aforesaid limits if densities greater than 3 gm/cc are required.

(Comparative example-influence of the quantity of MgO).

Again for comparison, the tests described under example 4 were repeated, but using MgO quantities of 2% and 1 % respectively. After sintering for 15 minutes at 17501C, densities of 2.93 and 5 2.66 g/CM3 respectively were obtained, whereas values of 3.15 and 3.13 g/cml had been obtained for 5% and 3% MgO respectively (see examples 2 and 4). These tests demonstrate that at less than 3% MgO, optimum densification is not obtained.

Claims

1. A silicon nitride-based sintering composition in powder form containing magnesium oxide and 10 aluminium oxide as densification aids, the particle size of the S6N4 not exceeding 1 g, and the particle size of the said densification aids being less than the particle size of the S13NO characterised in that the total amount of said densification aids does not exceed 6% by weight of the composition, and the MgO:A1,0, weight ratio is from 10: 1 to 1:3.

2. A composition as claimed in claim 1, in which the S1,1\14'S chosen from the a-, P- and amorphous varieties or their mixtures.

3. A composition as claimed in claim 1 or 2 which comprises 94% by weight of Si3N4 having a particle size of 0.5 Am, 5% of MgO having a particle size of 0.05 Am, and 1 % of finely ground A1103, all having been mixed intimately in a ball mill.

4. A method of manufacturing S6N4 objects by pressureless thermal sintering, comprising the 20 steps of:

a) compacting a powdered composition as claimed in any of the preceding claims, in the cold state, into the form of the required object, b) subjecting the preformed object in the cold state to an isostatic pressure of at least 1 T/cm', c) degassing the object pressed in the cold state by heating under reduced pressure, and 25 d) heating the degassed object for 2 to 20 minutes between 1650 and 18301C in an atmosphere composed essentially of nitrogen, so as to obtain the required sintering and densification.

5. A method as claimed in claim 4, in which the composition contains aS6N4 and step d) comprises heating for 15 minutes at 1750c1C.

6. A sintered product obtained by the method claimed in claim 4 or 5, having a density which 30 exceeds 3.10 and of which the bending strength at 250C is from 70 to 94 kg/mml.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1980. Published by the Patent Office, Southampton Buildings, London, WC2A l AY, from which copies maybe obtained.

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